1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more specifically to a method for forming an oxide film on a semiconductor substrate by an improved LOCOS method.
2. Description of the Prior Art
When a plurality of elements are formed on one semiconductor substrate in a semiconductor integrated circuit device, a well-known local oxidation of silicon (LOCOS) method is used in order to form an insulating film for separating element forming regions from one another.
However, since bird's beaks are produced in a silicon oxide film formed by the LOCOS method, the proportion of an area occupied by an isolation region becomes larger, where prevents micronization of the semiconductor device.
Therefore, in order to suppress a spread of bird's beaks and make the proportion of the area occupied by the isolation region smaller a polycrystalline silicon buffer LOCOS method (a PBL method) has been put to practical use. The PBL method is described in the reference documents shown hereunder for instance.
1 J. W. Lutze et al.: Bxt. Abstr. 175th Electrochem. Soc. Meeting, Vol. 89-1 (1989) pp. 289-290 PA1 2 W. D. Su et al.: Proc. Tech. Papers 1991 Int. Symp. VLSI Technology, Systems and Applications (IEEE Catalog No. 91TH0368-1) pp. 43-46
A method for forming a silicon oxide film by a PBL method according to a conventional example will be described hereinafter with reference to FIG. 1A to FIG. 1D.
First, as shown in FIG. 1A, a pad oxide film 2 is formed on a silicon substrate 1, and a polycrystalline silicon film 3 is formed thereafter on the pad oxide film 2. A silicon nitride film 4 is formed on the polycrystalline silicon film 3 immediately after the polycrystalline silicon film 3 is formed.
Then, as shown in FIG. 1B, the silicon nitride film 4 is selectively removed to expose the polycrystalline silicon film 3 in the region where a silicon oxide film for isolation is to be formed.
Next, as shown in FIG. 1C, the silicon substrate 1 is heated in an oxidizing atmosphere and the polycrystalline silicon film 3 and the silicon substrate 1 thereunder are oxidized selectively with a silicon nitride film 4a as a mask.
Thereafter, as shown in FIG. 1D, as removing the remaining silicon nitride film 4a and a polycrystalline silicon film 3a, an isolation region 6 with a silicon oxide film 5 and an element forming region 7 adjacent to the isolation region 6 are formed.
When the isolation region 6 is formed so as to form the slender belt-shaped element forming region 7 as shown in FIG. 7, however, portions where the surface looks rough have been observed markedly at projecting end portions of the element forming region 7 adjacent to recessing portions 10a and 10b of the isolation region 6. When the element sections of those portions are investigated, it has been found that anomaly of bird's beaks 9a and 9b had arisen by difference in level of the bird's beaks 9a and 9b as shown in FIG. 1C.
And also when an H-shaped element forming region 7a is formed as shown in FIG. 2, a phenomenon similar to the above also arisen in recessing portions 11a to 11d of an isolation region 6a.
When the anomalous silicon oxide film 5 for isolation having such difference in level is covered with an insulating film, a gate electrode or the like in a latter process, there is involved such a risk that formation of the insulating film, the gate electrode or the like becomes incomplete.